Anodic passivation of metals



Oct. 3, 1967 MEKJEAN ANQDIC PASSIVATION OF METALS Filed March 25, 1963United States Patent This invention relates to the prevention ofcorrosion of metals. More particularly, it relates to the prevention ofcorrosion of metals in contact with a molten salt system and to apassivation system relating thereto.

Molten salt systems have great utility throughout the chemical industry.They may be employed as fairly high constant temperature heat sourcesfor reactions, usually for cracking, reforming, disproportionating,halogen-ating, dehydroha'logenating, and catalytic reactions, includingspecific organic transformations, metatheses and recombinations. Thesesystems a'hve also been found useful in electrolytic reductions ofrefractory metal salts to the metal, such as in the production oftitanium, zirconium, tantalum, and niobium.

The alloy steel industry makes use of molten salt systems to assist inthe conversion, and subsequent removal, of heat scale developed inannealing and hot rolling mill operations. These systems are generallytermed descaling baths which fall into three main classifications. Theyare classified as (a) oxidizing, (b) reducing, and (c) neutral baths.These descaling baths are usually alkaline baths, e.g., molten causticbaths, and the type of additives incorporated with the caustic mel-tdetermines the classification of the bath. If sodium nitrate, anoxidizing agent, is added to the caustic melt, an oxidizing bath isformed, whereas a reducing agent additive produces a reducing bath. If aneutral agent (one neither oxidizing nor reducing in action) is added,e.g., sodium sulfate, a neutral bath is formed.

' New uses for molten salt systems are also being developed in thegrowing of crystals and of gem stones, for end uses in the fields ofelectronics, optics, electronic analyzers, jewelry, machine tool tips,phonograph styli and abrasives.

' The use of molten salt systems as a heat capacity heat sink or storageunit for home, office, or industrial heating is another area whichpresents unusual possibilities. By using lower cost electric powerduring periods of low demand, a caustic molten salt system may becapable of storing the total heat required for each 24-hour day. Usingthis system, the stored energy may be transferred to and then conveyedby hot water pipes, hot air ducts, or hot water radiators to the desiredarea.

In any molten salt system, however, an ever present problem is thecorrosion of the component elements, especially those made of metal. Thefrequent replacement of equipment necessitated by this problem is notonly costly, but at times is extremely inconvenient.

It is, therefore, an object of the invention to provide a method ofprotecting metals, in situ, from the corrosive action of molten saltsystems.

It is a further object of this invention to create and maintain asubstantially insoluble surface'film on metallic objects and equipmentto protect them from corrosive media to which they are subjected.

It is still a further object of this invention to use a novel type ofelectrolytic passivation'to protect metals from cor rosion in ananhydrous molten salt system.

In accordance with this invention, it has been found that the metals ina system which are in contact with a substantially anhydrous moltencorrosive salt bath may be passivated by applying electrical potentialbelow the decomposition voltage of the molten salt system on the metalto be passivated and thereafter maintaining the voltage below the saiddecomposition voltage of the electrolytes present.

The various aspects of the invention will be apparent from the followingdescription, taken with the drawing illustrative of an embodiment of theinvention, in which:

FIGURE 1 is a vertical section of the apparatus of the invention along11, and

FIGURE 2 is a horizontal view of the apparatus.

Referring to FIGURE 1, metal container 10 containsv a molten salt bath12, sludge pan 14, gas fired immersion tubes 16, fired by gas manifold18 and air manifold 20. The container 10 is equipped with jigs 22 tohold articles to be inserted into the molten salt bath. Duringoperation, the molten salt bath is maintained at the desired temperatureby the gas fired immersion tubes. Cathode 28 is inserted into the moltensalt bath. Direct current is supplied to cathode 28 and anode'contactmeans 30 by a source of electric current 32, the electrical lines beingschematically illustrated.

A metal is said to be passivated when its surface has been renderedcomparatively, substantially, and preferably, completely inactive to itsenvironment. It has been found that metals in contact with asubstantially nonaqueous molten salt system may be passivated accordingto this invention in either one of two ways, depending on thecompostiion of the molten salt system and the metal incontact with thesystem; The metal may be made the anode, causing oxidation to take placeat its surface to form a substantially insoluble protective film on themetal, or the metal may be made the cathode, causing reduction to takeplace at its surface and preventing the repeated development of solublefilms on the metal. This latter procedure passivates certain metals,which, if made the anode, might have a soluble film formed on itssurface. Such a film could continually dissolve in the molten saltsystem, causing the metal of the container or other part to corrode atan accelerated rate. If the metal is'maintained cathodic, the continualformation of a soluble film is prevented, thus passivating the metal tothe molten salt system.

The passivating system disclosed herein may be useful in passivatingmetals in contact with an alkali metal hydroxide salt bath containingoxidizing agents, reducing agents, neutral agents, or mixtures thereof.It may also be useful in passivating metals in contact with variousmolten anhydrous salt bath mixtures, such as alkali metal molten saltsystems, e.g., sodium and potassium fluoride or other alkali metalanhydrous salt systems; alkaline earth metal salt system-s, e.g.,calcium chloride, magnesium sulfate; and any combination of salts usefulas a molten salt system, e.g., sodium sulfate and barium chloride. It iseven within the contemplation of the invention that a metal in contactwith pure molten metal such as sodium in the presence of an anionselected from the group consisting of hydrides, sulfates, nitrates,persulfates, permanganates, sulfides, phosphates, carbonates, peroxides,chromates, sulfites, thiosulfates, halides, etc., may be passivated byforming a substantially insoluble film on its surface by the techniquedescribed herein. The alkali metals which may be suitably utilized inthe invention are sodium, potassium, lithium, rubidium, cesium andfrancium. The suitable alkaline earth may beselected from the groupconsisting of beryllium, magnesium, calcium, strontium, barium, andradium.

The passivation system as described herein is preferably utilized in amolten salt system that has, as its major constituent, sodium hydroxidewith the remaining proportion of the system made up of anions containingelectrolytically available oxygen. The anions which may be utilized inthe preferred salt system in combination with Patented Oct. 3, 1967-hydroxyl ions may be selected from the group consisting of nitrates,persulfates, permanganates, sulfides, sulfates, phosphates, carbonates,peroxides, chromates, sulfites, thiosulfates, hydrides, etc., all of thecombinations of which have electrolytically available oxygen so that anoxidation reaction may take place at the surface of the metal in contactwith the molten salt system to form a substantially insoluble film onthe metal surface.

Although electrolytically available oxygen is preferred in forming theinsoluble film on the metal surface, it may be desirable in a saltsystem to require the formation of a substantially insoluble film whichis not an oxide of the metal. For example, in a salt system where theavailable anion is fluoride ion, it is preferred to form an insolublemetal fluoride film on the surface of the metal and thus, passivate themetal.

In the preferred molten salt system of sodium hydroxide and sodiumsulfate, the preferred proportion of sodium hydroxide to sodium sulfatemay be 55 to 99 percent by weight sodium hydroxide and 1 to 45 percentby weight of sodium sulfate, with more preferred proportions being from80 to 99 percent by weight sodium hydroxide and 1 to 20 percent byweight sodium sulfate. The most preferred proportion is about 90 percentsodium hydroxide and 10 percent sodium sulfate.

All metals used in the molten salt system, such as the container pot,the heating elements, sheathing of the heaters and probes, heatingtubes, thermocouple probes, bafile plates, pumps, sludge pans,agitators, heat exchangers, electric resistance heaters, immersionrollers, quiet zones or sludge settling areas, sparger tubes andsubmerged gas lines, and any other equipment exposed to the molten saltsystem, will be passivated by the technique of the invention. Thepreferred metal used in molten salt systems varies with the salt systemutilized. Therefore, each system should be evaluated prior todetermining the metal to be used in the system. The determination is onewhich includes economic considerations and the result desired to beachieved. A technique that may be used is maintaining all the metalelements anodic, thus forming a protective, substantially insolublefilm, usually oxide, on the surface of the metal, or alternatively,where suitable, the metal elements may all be maintained cathodic orsome may be maintained anodic and others cathodic. Metals which aregenerally used in molten salt systems may be used in practicing theinvention, such as nickel, silver, low carbon steel, e.g., American Ironand Steel Institute 1010, other ferrous alloys, such as stainless steeltypes 302, 304, 310, 316, 405, 410, and 416, Hastelloy B and Carpenter20, and the rare metals, e.g., titanium, zirconium, hafnium, niobium.For the preferred molten salt system of sodium hydroxide and sodiumsulfate, the metal preferred to be utilized in the system is a lowcarbon steel. The passivation technique preferred for this particularsystem is the formation of a substantially insoluble oxide film on itssurface.

The temperature range at which the described invention may be employedis from 25 degrees centigrade to 3,000 degrees centigrade, which in allcases will be below the melting point of the metal being passivated. Apreferred temperature is from 100 degrees centigrade to 1,000 degreescentigrade, and a more preferred range is between 250 degrees centigradeand 800' degrees centigrade. In the preferred salt system, thetemperature may be between about l70 degrees centigrade and about 650degrees centigrade. Variation in temperature within this range has noadverse effect upon the passivated metals.

It is to be understood that for the process and passivat-' ing system ofthis invention to be effective, the molten salt system is substantiallyanhydrous. A slight amount of water present would be decomposed byelectrolysis to hydro gen and oxygen gases and removed as such from thesystem. This may also be true when the molten salt system picks up waterfrom the atmosphere or by condensation.

when the temperature of the bath is lowered.

4 The maximum current required to passivate a metal in contact with amolten salt system depends on the decomposition voltages of the moltenelectrolyte or electrolytes present. The electrical resistance of themolten salt system and the distance between the electrodes determine thecurrent that flows in a system. The decomposition voltage of eachcomponent may be determined by utilizing the Gibbs-Helmholtz equation:

J=electrical equiv. of heat (the Joule constant) z=valence of the metalF =free energy of formation T=degrees Kelvin (absolute temperature) orby referring to known decomposition tables, as for example, thoseillustrated at pages 242 and 243 of Electrochemistry, volume 1,Creighton and Koehler, fourth edition (John Wiley & Sons), 1951. Thecurrent density on the metal to be passivated may be varied between0.0025 ampere .per square foot to a maximum of about 10 amperes persquare foot in the preferred sodium hydroxide-sodium sulfate molten saltsystem. The maximum current density, as stated above, will dependprimarily on the decomposition voltage of each molten salt system andthe electrode material utilized. In the preferred system, about 5amperes per square foot of current are applied, when utilizing azirconium cathode with a low carbon steel container as the anode. Atfirst, the current may floW readily, but within 2 to 10 seconds, it maydecrease to almost zero. At this time the current density may bereestablished by increasing the voltage until a steady-state equilibriumsituation is achieved, i.e., the current flow remains constant. At thispoint, it is found, in the preferred embodiment of the invention, that avoltage of 2.6 volts maintains a current of 5 amperes. It was found thatby maintaining such a current in the preferred embodiment the low carbonsteel container walls are very well passivated. Other current flows atvoltages below the decomposition point of the bath are also useful toobtain this effect. It is believed that the initial surge of currentcreates a monomolecular film of ferric oxide on the total surface of theanode immersed in the molten salt system. As the current continues toflow, the oxide film builds up to a thicker and thicker layer, creatinga greater and greater resistance, which continues to limit the currentflow. Once the thickness of the oxide film is established, the currentflow, if desired and if the decomposition voltage of the ionic liquid isnot exceeded, may be increased by increasing the voltage. In thepreferred embodiment of this invention, a voltage of from about 2.4 toabout 2.8 volts was found to be suitable to maintain the passivatingfilm on the surface of the vessel.

However, voltages between about 0.005 volt and 3.7 volts may be usefullyemployed, depending on the molten salt system, the temperature and theelectrodes utilized. Direct current or rectified alternating current isutilized in the practice of the invention.

As mentioned above, in the preferred embodiment of the invention,zirconium was used as a long-lived sacrificial cathode. Other cathodematerials, however, may be selected, according to their efficiencies,life in the molten salt system utilized, and cost. Among the metalswhich may be used as a cathode and which exhibited superior electricaland corrosion-resistant properties, are copper, nickel, silver,molybdenum, their common alloys, and copper or silver sheathed inzirconium.

These are the initial reactions and thereafter there are essentially noreactions at the anode or cathode, but the potential applied maintainsthe passive film. The

Foo F6 created at the anode is a substantially insoluble film whichpassivates the metal against the corrosive environment.

From the above description, it is readily seen that a low carbon steelcontainer may be passivated in a corrosive molten salt system comprising90 percent sodium hydroxide and 10 percent of another salt, such as NaSO Na S O Na S O Na S, and Na S 0 or mixtures thereof. (This is areducing system.) The vessel is made so the anode and zirconium (orother suitable material indicated above) may be utilized as the cathode.The reactions illustrated in Equations 1, 2, and 3 are believed to besubstantially the reactions which take place in this system to passivatethe metal container.

An aluminum container holding a mixture of sodium hydroxide and sodiumsulfate may also be passivated, following the description of thisinvention, by making the container wall anodic. A surface him may becreated on the aluminum which is substantially insoluble. The reactionswhich are thought to take place at the anode and cathode of this system,utilizing a zirconium cathode, are as follows:

At the cathode:

In this case, the decomposition voltage is not exceeded and molecularoxygen is not released. A substantially insoluble aluminum oxide and/ orsulfate film is created and maintained at the anode, that passivates themetal and acts as an electrical insulator. Thus, little current isrequired to maintain the film after the initial flow of current. Thiselectrolytically-created and maintained oxide and/or sulfate film issubstantially non-soluble in the molten salt system, preserving thealuminum container from catastrophic corrosion which would normally beencountered.

The low carbon steel walls of a vessel with a molten salt alkalinehydroxide oxidizing bath therein may also be passivated and maintainedin a passive state in accordance with this invention. A molten saltsystem of sodium hydroxide, sodium nitrate and sodium nitrite, althoughessentially not exceedingly corrosive to steel, gradually undergoeschemical change, absorbing CO from the atmosphere and creating sodiumcarbonate at elevated temperatures such as those greater than 700degrees centigrade. The sodium nitrate decomposes to the nitrite andoxygen, which in turn continues to decompose to sodium monoxide andnitrous oxide. While the nitrate is present, it chemically creates apassive, insoluble film on the surface of steel. When thenitrate-nitrite mix is gone, and replace by sodium monoxide in the melt,a very corrosive system is established. By imposing an anodic current onthe steel container, at an elec-tromotive force not exceeding thedecomposition voltages of NaO NaNO or 6 NaNO the components of the mix,a passive iron oxide film is created and maintained, protecting thevessel.

As mentioned hereinabove, these are some molten salt systems andcontainer combinations wherein the film formed on the container walls,if they are made anodic, is more soluble in the melt than the base metalitself. For example, a caustic alkali melt containing an oxidizing agentin a silver vessel will form a soluble film on thesilver surface andcause corrosion of the base metal. To prevent the formation of a solublefilm the container walls may be made cathodic and a current applied, inaccordance with the description of this invention, to prevent theformation of such a soluble film. The base metal in this instancewould'have the greater resistance to dissolution than the metal with theoxide film forming thereon.

It will be recognized by those skilled in the art that variousmodifications within the invention are possible, some .of which arereferred to above. Therefore, the invention is not to be interpreted aslimited to the following examples. All values in the following examplesare by weight and temperatures are by degrees centigrade unlessotherwise stated.

Example I To a low carbon steel vessel was added 3,000 parts of sodiumhydroxide and 570 parts of sodium sulfate. The container was made anodicand a zirconium electrode was made cathodic and immersed in the saltmixture. The mixture was then heated to about 360 degrees centigrade.The current was initially established at 5 amperes at 2.6 volts,impressed and was maintained for 432 hours. At that time, the currentwas deliberately reduced to 2.5 ampers at 2.2 volts, and this wascontinued for an additional 215 hours. At the end of this period, aftera total elapsed time of 647 hours, there was essentially no evidence ofcorrosion. A thin film .of approximately 4 inch thickness was noted onthe inner walls of the container and the weight of the container haddecreased, from the 3445 parts it initially weighed, by only 7.5 parts.

Example II The experiment of Example I was followed up by reestablishingall the identical conditions of salt composition and temperature in thesame steel pot container, but without any of the present electrolyticpassivation techniques being applied. After 187 hours, the evidence ofcorrosion was so apparent that the experiment was terminated and thesituation evaluated. The container was found to have lost weightequivalent to 65.5 parts from an initial weight of 3445 parts. Had theexperiment been allowed to continue for the same total time as that ofExample I, or 647 hours, and assuming that the rate of corrosion wasuniform, the container would have lost 227 parts by weight, or over 30times the weight loss suffered without passivation. It may be seen,therefore, that the electrolytic passivation technique employed inExample I is effective in reducing corrosion by a favorable factor of atleast 30 to 1.

Example III Two steel heating tubes were immersed in a substan tiallyanhydrous fused salt mixture containing 2160 parts of sodium hydroxide,216 parts of sodium sulfate, and 24 parts of a combination of Na S O NaS O Na S O Na SO and N18. The fused salt mixture was heated to atemperature of 425 degrees centigrade. One of the steel heating tubeswas made anodic while the other was not connected to any electricalsystem. These tubes remained immersed in the fused salt mixture for atotal of about 1400 hours. About 17 percent of this time, the bath washeld at temperatures between about 625 and 675 degrees centrigrade. Formost of this time, the voltage impressed on the anodic steel tube was1.5 volts at a rectifier employed and 0.9 volt between the tube andcathode, resulting in a current of 2.5 amperes. A zirconium cathode wasutilized in this example. For a part of the time, the voltage applied tothe passivated tube was 0.05 to 0.1 volt. The average rates of corrosionwere then measured for the passivated and unpassivated tubes. Theaverage metal loss on the passivated tube from .one wall was 0.00011 7Example IV A salt bath melt having the composition of Example III wasagain utilized. This melt was held at 380 degrees centigrade. A steeltube was immersed in the bath and 2.7 volts was applied, making the tubeanodic. A copper cathode was utilized in this system. The pot walls weremade anodic and a nickel diaphragm was placed in the pot, but was notelectrically connected to the passivation system. After 196 hours, thesystem was shut down and the nickel diaphragm was examined. It wasobserved that corrosion of the diaphragm was extensive but the pot wallsshowed no visible evidence of corrosion.

A new nickel diaphragm was then installed in the melt and connected tothe passivation circuit of the pot. A voltage of 1.2 volts wasmaintained between electrodes. A current density of 0.00508ampere/square inch was employed.

After a total of 362 hours for the pot, and 166 hours for the newdiaphragm, the nickel diaphragm and the pot wall-s were visuallyexamined. Neither the pot walls nor the diaphragm was visibly attackedby the melt..

Example V A salt melt of the composition set forth in Example III wasagain charged to a steel pot and a zirconium diaphragm was connected tothe anodic passivation circuit. A copper cathode was utilized in themelt to complete the circuit.

The temperature of the melt was varied between 450 and 480 degreescentigrade, and 3 volts were impressed, which maintained 2 amperes inthe passivation circuit. An average current density of 0.002015ampere/square inch was employed. After 21 days, the steel pot showed novisible evidence of corrosion. The zirconium diaphragm also did not showany visible evidence of corrosion. Upon a weighing of this diaphragm,the visible observation was confirmed, as the weight remained constant.

Example VI A composition containing 95 percent NaOI-I and 5 percent NaPOgwas charged into a steel pot. A steel baffie plate, a steel housingand a sludge pan with lifting rods connected thereto, were immersed inthe melt. The temperature of the melt was held at between about 450 and480 degrees centigrade. The baflie plate was completely corroded after28 days. At this time, the other equipment was visually examined. Allelements showed signs of ex cessive corrosion and a large volume ofoxide was collected within the sludge pan, indicating rapid corrosion ofthe elements.

' Example VII Example VI was repated, utilizing a nickel diaphragm and apassivation circuit in addition to the equipment utilized in Example VI.The anodic passivation circuit was connected in parallel to all theelements, and a copper cathode completed the circuit. Initially 3.75volts was applied between the electodes but this was gradually decreasedto 0.9 volt. After six and half months, the system was shut down and allelements were visually inspected. There was no evidence of corrosion ofany of the metallic elements connected to the passivation circuit.

8 Example VIII Equipment similar to the equipment utilized in Example VIwas placed into a molten salt composition of V percent sodium hydroxide,10' percent potassium chloride, and 5 percent sodium phosphate. Acurrent of 3 amperes was applied to the passivation circuit with 1.8volts at the rectifier, and 0.9 volt across the electrode.

After about three months, the salt was pumped out and the equipment wasfound to be in perfect condition, with no visible evidence of corrosion.

Example IX Example VIII was repeated utilizing a molten salt compositionof percent sodium hydroxide, 1 percent sodium chloride, 1 percent sodiumcarbonate and 8 percent sodium nitrate. The same current and voltage ofExample VIII were again applied for about three months. Again there wasno visible evidence of corrosion of the elements.

The invention has been described with respect to preferred embodimentsthereof, but is not to be construed as limited thereto. Variations ofthe invention may be made and equivalents may be substituted thereinwithout going beyond the invention or the scope of the claims.

What is claimed is:

1. A method for anodically passivating a metallic element in contactwith a substantially anhydrous molten com-position comprising contactinga substantially anhydrous molten composition comprised of at least 55percent alkali metal hydroxide with a corrodible metallic element to beprotected selected from the group consisting of steel, nickel, silver,titanium, zirconium, hafnium and niobium, contacting said anhydrouscomposition with a second metallic cathode element comprised of a metalselected from the group consisting of zirconium, copper, nickel, silverand molybdenum, applying a voltage of about 0.005 to 3.7 volts betweensaid corrodible metallic element and said cathode element, said voltagebeing below the decomposition voltage of the anhydrous composition andsufficient to make the corrosive metal anodic and the second metallicelement cathodic, thereby passivating said corrodible metallic elementto the action of said substantially anhydrous composition.

2. The method of claim 1 wherein the temperature of the anhydrouscomposition is maintained between about the melting point and 3,000degrees centigrade.

3. The method of claim 1 wherein the alkali metal hydroxide is sodiumhydroxide.

4 The process of claim 1 wherein the cathode is zirconium.

5. The method of claim 1 wherein the metal to be protected is steel.

6. A method for anodically passivating a metallic element in a metalliccontainer containing a substantially anhydrous sodium hydroxidecomprising contacting a substantially anhydrous sodium hydroxidecomposition with a steel element to be protected at a temperature of 250to 800 degrees centigrade, contacting said anhydrous composition with azirconium cathode, applying a voltage of 0.005 to 3.7 volts between saidsteel element and said cathode, said voltage being below thedecomposition voltage of said sodium hydroxide composition, therebymaking said steel element anodic and passivating said steel element tothe action of said substantially anhydrous sodium hydroxidecompositions.

References Cited UNITED STATES PATENTS 476,914 9/1892 Bernard 204-196641,438 1/1900 Darling 204l96 1,507,395 9/1924 Mershon 204-496 1,545,3847/1925 Ashcroft 204196 2,311,257 2/1943 Sawyer et al 204196 2,485,27610/1949 Gerbes 204-496 9 7/ 1951 Peyches 204-147 10/1959 Nies 2041478/1965 MacNab 204147 FOREIGN PATENTS 2/1962 Great Britain.

10 OTHER REFERENCES

1. A METHOD FOR ANODICALLY PASSIVATING A METALLIC ELEMENT IN CONTACTWITH A SUBSTANTIALLY ANHYDROUS MOLTEN COMPOSITION COMPRISING CONTACTINGA SUBSTANTIALLY ANHYROUS MOLTEN COMPOSITION COMPRISED OF AT LEAST 55PERCENT ALKALI METAL HYDROXIDE WITH A CORRODIBLE METALLIC ELEMENT TO BEPROTECTED SELECTED FROM THE GROUP CONSISTING OF STEEL, NICKEL, SILVER,TITANIUM, ZIRCONIUM, HAFNIUM AND NIOBIUM, CONTACTING SAID ANHYDROUSCOMPOSITION WITH A SECOND METALLIC CATHODE ELEMENT COMPRISED OF A METALSELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM, COPPER, NICKEL, SILVERAND MOLYBDENUM, APPLYING A VOLTAGE OF ABOUT 0.005 TO 3.7 VOLTS BETWEENSAID CORRODIBLE METALLIC ELEMENT AND SAID CATHODE ELEMENT, SAID VOLTAGEBEING BELOW THE DECOMPOSITION VOLTAGE OF THE ANHYDROUS COMPOSITION ANDSUFFICIENT TO MAKE THE CORROSIVE METAL ANODIC AND THE SECOND METALLICELEMENT CATHODIC, THEREBY PASSIVATING SAID CORRODIBLE METALLIC ELEMENTTO THE ACTION OF SAID SUBSTANTIALLY ANHYDROUS COMPOSITION.